Refactoring repo urls.

Modifying how repo urls are computed and used by the recipes to allow
easy overloading of urls for specific components. This is mostly
useful for managing installs from various internal sources for
testing.
Remove the default for Openstack version attribute.

There exists no reasonable defaults for the openstack version
attribute and defaulting to the wrong version can result in a
seemingly working installation that's actually subtly broken. Instead,
we now provide no default values and raise an error if no version is
given.
Merge pull request #11 from rmahmood/independent-urls

Refactor repo urls to allow specification of independent urls for each component.